1. Field of the Invention
The invention relates to a tunable impedance matching circuit and particularly relates to a tunable impedance matching circuit having a tunable inductor.
2. Description of Related Art
Impedance matching is important for electronic devices that emit wireless signals, such as mobile phones. If an RF (radio frequency) signal encounters unmatched impedance when being transmitted from a power amplifier to an antenna, a portion of the signal energy will be reflected by the antenna, and consequently the signal energy output by the power amplifier cannot be completely emitted by the antenna. Therefore, impedance matching is an important issue in the field of wireless communication.
A mobile phone usually includes an impedance matching circuit for matching the impedance of the antenna, so that the energy emitted from the power amplifier can be completely transmitted to the antenna to achieve the best transmission efficiency. The impedance of the antenna may vary when affected by nearby objects. Moreover, now most mobile phones support multi-band communication. Take GSM (Global System for Mobile Communications) as an example, GSM 900, GSM 1800, and GSM 1900 use different frequency bands, and for this reason, mobile phones that support the foregoing need to perform impedance matching in different frequency bands. These factors increase the complexity of impedance matching, and fixed impedance matching circuits can no longer satisfy the demands. Tunable impedance matching circuits provide wider matching ranges and thus are suitable for complicated application that requires multiple frequency bands.
FIG. 1 is a schematic diagram of a conventional tunable impedance matching circuit 100, wherein a voltage source 102 supplies a DC bias voltage, and a signal source 101 provides an RF signal to an antenna 103, which emits the RF signal. A tunable capacitor 104 changes a capacitance thereof according to a control signal, thereby changing the impedance encountered by the RF signal, so as to match the impedance of the antenna 103. In the tunable impedance matching circuit 100, only the series path has tunable impedance, and the impedance of the shunt path is fixed. Therefore, the tunable impedance matching circuit 100 has a smaller matching range, which may not be sufficient.
FIG. 2 is a schematic diagram of another conventional tunable impedance matching circuit 200. The series path of the tunable impedance matching circuit 200 includes a tunable capacitor 202, and the shunt path thereof includes tunable capacitors 201 and 203. In the tunable impedance matching circuit 200, both the series path and the shunt path have tunable impedances, which provides a wider matching range. However, in order to widen the matching range, the tunable capacitor 202 in this type of impedance matching circuit is usually designed to have a wider impedance-tunable range, e.g. from 0.7 pF to 7 pF, that is, a tunable impedance range of 1:10. The capacitor with the wider tunable range will increase consumption of the RF signal and impair the over-the-air performance of the antenna.